1. Field of the Invention
The invention relates to gas-liquid mixing operations. More particularly, it relates to enhanced gas-liquid mixing under particular variable liquid level operating conditions.
2. Description of the Prior Art
In gas-liquid mixing operations, the Advanced Gas Reactor (AGR) system employs a down-pumping impeller positioned within a hollow draft tube to create a recirculating flow pattern in a body of liquid contained in a mixing vessel. Because of such recirculation of the liquid downward in the hollow draft tube, vortices are formed in the upper inlet area of the draft tube to create a suction which draws gas from an overhead gas space within the vessel and mixes it with the recirculating liquid passing downward into the draft tube, as disclosed in commonly-assigned Litz U.S. Pat. No. Re. 32,562.
Satisfactory vortex development for such gas-liquid mixing purposes depends, among various practical operating factors, on maintaining a proper liquid level range above the top of the draft tube. If the liquid level rises above this narrow operating range, the formation of vortices is impeded, and the gas ingestion rate decreases essentially to zero. Thus, operating at liquid levels above the optimum level can substantially reduce the gas ingestion capabilities of the AGR system. Also, if the liquid level falls below the top of the draft tube, all gas suction ceases.
The AGR does not work properly when the liquid level in the reactor is greater than about 1/3 to 1/2 draft tube diameter above the draft tube. The impeller becomes liquid flooded such that the vortex entrainment mechanism, which is the primary means for drawing gas into the AGR impeller suction, is effectively cut off. The AGR is not useful for this reason. Variable level applications are common in specialty chemical processing, particularly in hydrogenation and chlorination reactors. Successful adaptation of the AGR for use in the variable level reactors will allow benefits that have been observed in fixed level AGR applications to be realized in the variable level applications.
There are currently three methods concerned with overcoming the problems associated with using the AGR in variable level applications. The first, described in U.S. Pat. No. 5,009,816, uses two or more AGR impeller/draft tube sets placed one above the other. The impellets are mounted on a common shaft. The draft tubes are spaced to allow flow from the bulk liquid into the suction of each impeller. The lowest impeller/draft tube is positioned to operate normally at the lowest operating liquid level. As the liquid level is increased during a batch, the next higher impeller/draft tube set becomes submerged and thereby becomes activated. Thus one of the impeller/draft tube sets is always running at or near the liquid surface, such that the uppermost active impeller/draft tube set in never liquid flooded and the vortex entrainment mechanism of gas ingestion is effective at all liquid levels. The multiple AGR concept described in U.S. Pat. No. 5,009,816 is mechanically complex and expensive to implement. In a second method, described in commonly-owned U.S. Pat. Nos. 4,919,849 and 5,244,603, the AGR impeller shaft is made hollow, and a hold is drilled in the shaft at a position above the maximum liquid level in the reactor. Hollow educator tubes are attached to the shaft in the vicinity of the impeller suction. As the impeller rotates, a negative pressure is generated at the tips of the eductor tubes. The pressure differential between the gas space where the hole is located, and the eductor tips, causes gas to flow from the gas space through the shaft to the impeller suction.
In Litz U.S. Pat. No. 4,919,849, the use of hollow gas ingestion tubes connected to a hollow shaft is disclosed as a means for drawing gas into downward pumping helical impeller means located at non-optimum liquid levels during the course of gas-liquid mixing operations subject to variable liquid level operations. In many gas/liquid mixing applications, particularly those in the specialty chemical and pharmaceutical areas, variations in liquid level within a vessel are very common. They may be caused by variations in the batch size processed, an increase or decrease in the volume of reactants consumed or dissolved, or the addition or removal of material as the reaction proceeds. In many processes, it is desirable to be able to recirculate a gas or gases that accumulate in the vessel head space. This is particularly the case in hydrogenation and oxygenation processes. While the above-indicated Litz U.S. Pat. No. 4,919,849, addresses this matter and provides for the drawing of gas from the overhead gas space in circumstances in which the vortex development of an AGR system, and thus gas ingestion, is impeded, further improvements are desired in the gas-liquid mixing art. In particular, it is desired to provide for enhanced gas-liquid mixing in reactor vessels having very large liquid variations, e.g., as much as 8 feet or more, during the course of gas-liquid mixing operations. The impeller/eductor tube concept of U.S. Pat. Nos. 4,919,849 and 5,244,603 works well as long as the pressure differential generated by the rotating eductor tubes is greater than the liquid head above the eductor tubes. This can be a problem when the liquid level varies widely because the liquid head can be substantial. In the impeller/eductor tube system, the differential pressure which drives gas from the gas space to the impeller suction increases as N.sup.2, where N is the rotational speed of the impeller. Thus the range of acceptable operation can be increased by increasing the rotational speed of the impeller. However, impeller power draw increases approximately as N.sup.3. At high liquid level, high rotational speed is required to overcome the liquid head, so power draw can be excessive. Furthermore, when the liquid head is high, the required rotational speed can approach the impellet's critical speed, which can cause severe vibration and increases the likelihood of mechanical problems.
Another method for overcoming the problems associated with variable level applications, described in commonly-owned U.S. Pat. No. 5,004,571, uses an external surge tank and level control to maintain the liquid level in the reactor vessel at or near the optimum level for conventional AGR operation. The primary mechanism of gas dispersion is by vortex entrainment of the gas from the gas space into the impeller suction.
The external surge tank method requires controls to maintain the optimum operating liquid level, an additional pressure vessel, and pressure and level controls. Thus the system is complex and expensive.